The invention relates generally to a non-intrusive and in situ laser position sensing method and apparatus and, in particular, to a method and apparatus for accurately locating the angular position of a rotary actuator arm of a disk drive, in which a small reflective notch is formed on the actuatar arm, or a flat reflecting surface is attached to or formed integral with the actuator arm.
The invention provides an improved technique for writing precision servo tracks on the recording medium of the disk drive without requiring the usual push-pin mechanism of the prior art, or the need to mount a heavy retro reflector on the rotary actuator arm.
The method and apparatus of the invention also have other applications such as the measurement of precise rotational angles, rotary encoders, the precise control of a rotary actuator, and angular positioning in conjunction with a laser interferometer.
Present day high capacity magnetic disk drives use, for example, 10,000 tpi (tracks per inch) or more, and optical disks often have ten times that track density. Reliable reading and writing of information on such closely-spaced tracks require precise read/write head positioning. Precise positioning systems require a position feedback signal from the disk drive. To provide such a feedback signal in a disk drive, tracks of position information are placed on the disk media by a servo-track writer in accordance with prior art procedures. For acceptable performance, only one percent of the total track spacing is usually allowed for the track writer position error. The small size of this allowable error, and the small size of the disk drives, make the non-intrusive and in situ laser positioning system of the present invention extremely important.
In general, a laser interferometer, such as described in Brown et al. Pat. No. 5,108,184, or a laser Doppler displacement meter (LDDM) such as described in Charles P. Wang Pat. No. 4,715,706, is used to measure the precision displacement of a target, which may be either a flat mirror or a corner cube mounted on the rotary actuator arm of the disk drive. The laser beam reflected from the target and returned to the receiver may be laterally displaced by a small amount, typically half the beam diameter, but the return beam should be parallel to the output beam within a few arc minutes. Prior art techniques require the attachment and subsequent removal of the corner cube from the rotary actuator arm. Therefore, in the prior art, the disk drive must be typically servo written before it is completely assembled. The presence of a relatively massive corner cube during servo writing affects the natural frequencies of the disk drive, and the subsequent removal of the corner cube and completion of the assembly of the disk drive after it has been servo written may cause distortions in the servo pattern and resulting track misregistration. Also, the use of mechanical linkage in the prior art, such as a push-pin, to determine the position of the actuator arm may significantly decrease the system accuracy and repeatability.
The Brown et al. Pat. No. 5,108,184 referred to above discloses a non-invasive laser positioning system for disk drive rotary actuator arms. A laser interferometer is used in the Brown et al. system to determine the angular position of a rotary actuator arm which has a fixed center of rotation, and which has a flat mirror mounted thereon. The laser beam reflected from the flat mirror on the actuator arm is intercepted by a fixed corner cube. The laser beam reflected by the corner cube, which is parallel to the incident beam, is reflected again by the flat mirror on the actuator arm toward the interferometer which receives the twice-reflected beam. Hence, the angular position of the actuator arm can be determined. However, as the flat mirror rotates in the Brown et al. system, the reflected beam also rotates at twice the angle of rotation of the flat mirror and the reflecting point also moves. Thus, in the Brown et al. system, the laser beam may miss the corner cube completely. Accordingly, proper operation of the Brown et al. system is possible only when the laser interferometer and the corner cube are mounted so that the corner cube is positioned at a convergence plane of the laser beams, and the carrier cub returns all reflected laser beams back to the flat mirror and hence to the interferometer. However, there are two main difficulties in the Brown et al. system, specifically, the convergence plane must be very close to the flat mirror on the actuatar arm, and a large laser beam diameter and a large area photo detector are required to allow for the large lateral movement of the return laser beam caused by the rotation of the flat mirror. These difficulties limit the application of the Brown et al. system insofar as servo writers are concerned.
It is accordingly an object of the present invention to provide a system and apparatus which serves to overcome the difficulties enunciated above.